。
She declared that she was right.
Literal
She [topic-は] herself [subject-が] right [quotative-と] declared.
断言する ('declare unequivocally, assert categorically') is markedly stronger than ordinary 言う or 主張する. 断 carries the sense of 'cut off, decide' (as in 判断 'judgment,' 決断 'decision'), and 断言 implies a statement made with no room for doubt. The Sino-Japanese (漢語) verb gives the sentence a formal, weighty register — appropriate for news reports, court testimony, or any context where finality matters.